The repair of mismatched bases on opposing DNA strains is an important cellular mechanism for protection against mutagens and carcinogens. Heteroduplex regions also arise during recombination. Preliminary studies suggest that some nonsense mutants in gene cI of bacteriophage lambda undergo mismatch repair (or gene conversion) more frequently than do missense mutants. Additional missense and nonsense mutations in lambda gene cI will be isolated. Recombination between cI mutants will be studied in Escherichia coli under conditions that allow phage DNA replication. The frequency of co-conversion of closely-linked markers will indicate the length of excised nucleotide tracts. The polarity of nonsense mutants will be compared by measurement of the expression of gene rex, which is distal to cI. Host mutations that influence the translation of nonsense codons, transcription termination, or UV repair will be studied for their effect on mismatch repair. The effect on gene conversion of a phage mutation that prevents transcription of gene cI will be investigated. The length of lambda heteroduplex regions will be estimated by observing the frequency of heterozygous recombinants in crosses between complementing clear-plaque mutants. A large fraction of heterozygous recombinants produce small plaques. The possibility that lambda DNA packaged during recombination contains nicks or gaps will be investigated by crossing phages in bacterial hosts defective in ligase or DNA polymerase.